Evidence from studies with HIV and SIV argues strongly that macrophage tropic variants and brain microglial cells play a central role in the manifestation of neurological disorders in AIDS. However, it is not clear whether generalized tissue macrophage tropic viruses manifest systemically in all organs or whether "neurotropic" variants are present in the affected brain and responsible for the neuropathogenesis of AIDS. The molecular mechanisms by which HIV/SIV enter the central nervous system and cause neurological dysfunction remain to be elucidated. The proposed studies will explore molecular mechanisms for the neuropathogenesis of AIDS using the SIV/rhesus macaque model system. We have isolated cell-type specific SIV variants from rhesus macaque Mm155 that displayed characteristic SIV encephalopathy and giant cell disease in lymph nodes. Rhesus macaque primary brain cultured cells were used to isolate SIVmac155/NT variant from the brain tissue of Mm155. SIVmac155/TT and SIVmac155/MT were isolated from lymph nodes of Mm155 by using rhesus macaque peripheral blood lymphocytes and macrophages, respectively. SIVmac155/TT exhibited strict T-cell tropism, but both SIVmac155/MT and SIVmac155/NT have shown macrophage tropism in cultures. The brain isolate SIVmac155/NT, however, exhibits different biological properties and a much stronger neurotropism compared to SIVmac155/MT in culture. Specific genetic changes were identified in the env gene of each SIVmac155 variant, indicating the presence of a SIV variant in the affected brain that is biologically and genetically different from peripheral macrophage tropic SIV. Furthermore, analysis of SIV env sequences directly from tissues of Mm155 revealed that i) SIVmac155/TT was specifically present in the lymph node, ii) SIVmac155/MT was present in both lymph node and brain tissues, and iii) brain isolate SIVmac155/NT was exclusively present in the brain tissue of Mm155. This indicated the complex nature of AIDS neuropathogenesis in which both brain specific variant and peripheral macrophage tropic variant are involved. In the proposed study, we will investigate the role of SIVmac155/NT and SIVmac155/MT in the genesis of neurological disorders using in vitro and in vivo systems. We will construct env recombinant cloned SIVs to demonstrate the role of env sequences in facilitating virus replication in brain tissues and to delineate the genetic determinants of neurotropism. Additional virus domain(s) contributing to neurotropism of SIV will be identified and fully-competent neurotropic cloned SIV will be generated. We will biochemically examine the mechanisms by which the virus envelope determines neurotropism. The significance of env for neuropathogenesis will be analyzed not only by in vitro systems but also by experimental inoculation of the recombinant cloned virus into rhesus macaques. The long-term objective of this proposal is to establish a rhesus macaque model system for the study of neuropathogenesis of AIDS using neuropathic cloned SIV. The system will no doubt greatly contribute to our knowledge of the neuropathogenesis of AIDS and could potentially be an excellent system for drug and vaccine development to prevent neurological consequences of HIV-infected patients.